1. Field of the Invention
The present invention relates to a disk drive and a beam angle adjusting structure, and more specifically, it relates to a disk drive and a beam angle adjusting structure preventing a non-smooth motion resulting from employment of screws for adjusting the entrance angle of a light beam applied to a disk.
2. Description of the Background Art
A disk drive for a compact disk (CD) or a mini disk (MD) has a beam angle adjusting mechanism adjusting a beam emitted from an optical pickup for applying the same to the surface of the disk at a prescribed angle (90° in general). A recent disk subjected to high-density recording has a bit interval narrowly set in the radial direction. In order to read a signal recorded in high density, the numerical aperture (NA) of an objective lens set on a beam outgoing position is increased for reducing the beam diameter. It is known that comatic aberration results in proportion to the cube of the numerical aperture and the thickness of the disk if the optical axis of a laser beam is even slightly inclined with respect to a signal surface of the disk. This comatic aberration distorts the waveform of a reproduced signal and causes factors for voltage fluctuation, instability of the mechanism and the like. Therefore, the beam angle adjusting mechanism is provided on a chassis for adjusting the inclination of the optical axis of the laser beam. The system of this beam angle adjusting mechanism varies with the maker or the type of the mechanism. In relation to an optical pickup sliding in the radial direction of the disk along a guide shaft, for example, the following systems are known, for example:
(a1) A single guide shaft is employed for adjusting inclination along the guide shaft (along the radial direction of the disk) by changing inclination of a turntable while adjusting inclination in a direction intersecting with the guide shaft (intersecting with the radial direction) by an angular position of the optical pickup, rotatably and movably engaging with the guide shaft, about the guide shaft.
(a2) Two guide shafts are provided for varying heights for fixing ends of the guide shafts on both ends of the two guide shafts, i.e., on four portions, and changing the angle of the optical pickup thereby adjusting the entrance angle of the light beam upon the disk.
There are many other adjusting systems in addition to the above. The beam angle adjusting mechanism having any of these systems is assembled into the disk drive in situ in a step of manufacturing the disk drive and set to a proper beam angle.
FIG. 8 illustrates a beam angle adjusting mechanism according to the aforementioned system (a2). Referring to FIG. 8, an optical pickup 102 comprising an objective lens 122 defining a beam outlet on its upper surface is guided by two guide shafts 104 to radially move along a disk 107 placed on a turntable 103. Both ends of the two guide shafts 104 are held between brackets 109 fixed to a chassis 101 with set screws 150 and supported by bolts 106 from under the chassis 101, to be adjusted in inclination. The guide shafts 104 are urged by springs 140 to approach the chassis 101, while the bolts 106 arranged under the brackets 109 adjust the height for holding the guide shafts 104. This holding height is so adjusted on four portions that inclination of the optical pickup 102 can be adjusted along the radial direction of the disk 107 and the direction intersecting therewith. Consequently, the beam angle adjusting mechanism can control the beam emitted from the optical pickup 102 to ensure a state perpendicular to the surface of the disk 107.
As shown in FIG. 9, each bolt 106 is fitted with an internal screw 110 provided on the chassis 101 for adjusting the holding height. A clearance S for thread engagement is necessarily provided between the threads of the bolt (external screw) 106 and the internal screw 110. After the bolt 106 is screwed into the internal screw 110 with a tool and adjusted when the beam angle adjusting mechanism is assembled into a disk drive, therefore, jolts may be formed between the external screw 106 and the internal screw 110 due to an impact caused when the optical pickup 102 slides along the guide shafts 104 or falls on the floor immediately after removal of a torque from the tool or after the adjustment. Such jolts result in inclination of the external screw 106 and fluctuation of the height for holding each guide shaft 104, as shown in FIG. 10. This fluctuation of the holding height immediately results in angle fluctuation of the laser beam, to bring the aforementioned voltage fluctuation and instability.
In order to sole the aforementioned problem, a spring section 108 may be provided in each bracket 109 fixed to the chassis 101 for unidirectionally applying urging force F to the external screw 106 thereby preventing jolts between the external screw 106 and the internal screw 110. The spring section 108 has a smooth pressing surface facing the external screw 106 to be in contact with a plurality of crests of the external screw 106. The external screw 106 and the internal screw 110 can be prevented from jolts due to this spring section 108, for sustaining the effect of adjustment.
When the spring section 108 is employed in the aforementioned manner, however, the number of components is naturally increased while the man-hour for arranging the spring section 108 in each bracket 109 in a prescribed posture is also increased. Therefore, development of a structure preventing screws for adjustment from jolts without increasing the number of components such as spring sections has been awaited.